Over the decades, wireless communication systems have become more and more technologically advanced, with performance increasing in terms of smaller size, operation at higher frequencies and the accompanying increase in bandwidth, lower power consumption for a given power output, and robustness, among other factors. The trend toward better communication systems puts ever-greater demands on the manufacturers of these systems.
Today, the demands of satellite, military, and other cutting-edge digital communication systems are being met with microwave technology.
Many of these systems use digital phase modulation, such as biphase modulation, to encode data onto the carrier wave. This requires modulators at the transmission end and demodulators (mixers) at the receiving end of a communication signal. In many microwave communication applications, a double-balanced microwave mixer is utilized as a biphase modulator. Examples of microwave mixers that are built for this purpose are disclosed in Maas, S., Microwave Mixers, 2nd Edition, Artech House, 1993.
It is preferable for biphase modulators, such as those operating at X-band or K-band frequencies, to have minimal phase and amplitude imbalance in order to eliminate errors in data transmission. There is also a need to isolate balanced and unbalanced circuitry while conveying signal information between them. Typical double-balanced microwave mixers and biphase modulators are composed of rings of Schottky diodes and baluns. The baluns are transmission line structures that transform an unbalanced system impedance to or from a balanced diode impedance, thereby isolating balanced and unbalanced circuitry while allowing signal information to travel between them. Typical balun designs used with biphase modulators are disclosed in Sturdivant, R., Balun Designs for Wireless, . . . Mixers, Amplifiers, Antennas, Applied Microwave, summer 1993.
The structure of a balun typically used for this application is a broadside coupled, suspended transmission line with a short-circuited quarter-wave, high impedance transmission line that is attached to the balanced end of the coupled line. Alternatively, two short-circuited lines, or shunt inductors, may be used at the balanced end of the balun as a shorting transformer. In either case, a short-circuited line provides a direct current return for the diode currents, but has the detrimental effect of decreasing the effective bandwidth of the balun if the length of the line is not carefully chosen and precisely manufactured, and also degrades the phase and amplitude balances of the modulator.
An effective biphase modulator has several attributes. It is important that the two binary states of the modulator are balanced. Ideally the amplitudes of the two states are equal to each other, and the phase difference is exactly 180 degrees. However, the precise balance of the amplitude and phase between the two binary states is limited by any asymmetry of the balun structure. The balance is also limited by stray reactances introduced by the ring of Schottky diodes.
In space or satellite applications, circuitry employing biphase modulators are often subjected to tremendous stress, such as the severe vibrations and mechanical shock experienced during a launch. A syntactic foam powder is often utilized to fill the cavity around the circuitry to prevent damage to the circuit components. It is apparent to those skilled in the art that the placement of foam around the circuitry exacerbates the effects of an asymmetric balun by introducing a material with a higher dielectric constant and loss tangent than air. The permittivity of the foam significantly increases the insertion loss of the modulator, and causes the phase and amplitude imbalance created by the asymmetric balun to become substantially worse.